The Pregnancy Substitute that Fanny must take is described as including Syrup of Corpeus Luteum, Ovarin, Mammary Gland Extract, and 5cc of Placentin.
Mammary glands are the glands in mammals that produce milk for their young. Placentin may refer to the placenta or to the hormones or proteins produced by it. The placenta is the organ that connects the developing foetus to the uterus, allowing nutrients to pass through to it and waste to pass out. The corpeus luteum is a temporary system of glands that develops in the ovary. If the egg is fertilised it releases hormones that will thicken the lining of the uterus, which is essential for maintaining pregnancy.
These are all natural to a woman's birth cycle. In Brave New World, women will never become pregnant and give birth, so must take these substitutes in order to 'fool' the body into feeling that it is carrying out its natural functions.